Automated photomask inspection apparatus

ABSTRACT

An automated photomask inspection apparatus including an XY state (12) for transporting a substrate (14) under test in a serpentine path in an XY plane, an optical system (16) comprising a laser (30), a transmission light detector (34), a reflected light detector (36), optical elements defining reference beam paths and illuminating beam paths between the laser, the substrate and the detectors and an acousto-optical beam scanner (40, 42) for reciprocatingly scanning the illuminating and reference beams relative to the substrate surface, and an electronic control, analysis and display system for controlling the operation of the stage and optical system and for interpreting and storing the signals output by the detectors. The apparatus can operate in a die-to-die comparison mode or a die-to-database mode.

This is a continuation of application(s) Ser. No. 07/748,984, filed onAug. 22, 1991 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to electro-optical inspectionsystems, and more particularly to an automated photomask inspectionapparatus for detecting defects on optical masks and reticles and thelike.

2. Brief Description of the Prior Art

Integrated circuits are made by photolithographic processes which usephotomasks or reticles and an associated light source to project acircuit image onto a silicon wafer. A high production yield iscontingent on having defectless masks and reticles. Since it isinevitable that defects will occur in the mask, these defects have to befound and repaired prior to using the mask.

Automated mask inspection systems have existed for over 15 years. Theearliest such system, the Bell Telephone Laboratories AMIS system (JohnBruning et al , "An Automated Mask Inspection System--AMIS", IEEETransactions on Electron Devices, Vol. ED-22, No. 7 Jul. 1971, pp 487 to495), used a laser that scanned the mask. Subsequent systems used alinear sensor to inspect an image projected by the mask, such asdescribed by Levy et al. (U.S. Pat. No. 4,247,203, "Automatic PhotomaskInspection System and Apparatus") who teach die-to-die inspection, i.e.,inspection of two adjacent dice by comparing them to each other.Alternately, Danielson et al. (U.S. Pat. No. 4,926,489, "ReticleInspection System") teach die-to-database inspection, i.e. inspection ofthe reticle by comparison to the database from which the reticle wasmade.

As the complexity of the integrated circuits has increased, so has thedemand on the inspection process. Both the need for resolving smallerdefects and for inspecting larger areas have resulted in much greaterspeed requirements, in terms of number of picture elements per secondprocessed. The increased demands have given rise to improvementsdescribed in a number of subsequently issued patents, such as U.S. Pat.No. 4,247,203, entitled "Automatic Photomask Inspection System andApparatus", Levy et al., issued Jan. 27, 1981; U.S. Pat. No. 4,579,455,entitled "Photomask Inspection Apparatus and Method with Improved DefectDetection" Levy et al., issued Apr. 1, 1986; U.S. Pat. No. 4,633,504,entitled "Automatic Photomask Inspection System Having Image EnhancementMeans" Mark J Wihl, issued Dec. 30, 1986; and U.S. Pat. No. 4,805,123,entitled "Automatic Photomask Inspection and Reticle Inspection Methodand Apparatus Including Improved Defect Detector and AlignmentSubsystem", Specht et al, issued Feb. 14, 1989. Also of relevance issome prior art in the wafer inspection area, such as U.S. Pat. No.4,644,172, entitled "Electronic Control of an Automatic Wafer InspectionSystem" Sandland et al, issued Feb. 17, 1987.

Another force driving the development of improved inspection techniquesis the emergence of phase shift mask technology. With this technology itwill be possible to print finer linewidths, down to 0.25 micrometers orless. This technology is described by Burn J. Lin, "Phase-Shifting andOther Challenges in Optical Mask Technology", Proceeding of the 10thAnnual Symposium on Microlithography, SPIE,--the International Societyof Optical Engineering, Vol. 1496, pages 54 to 79.

The above improvements teach the automatic detection of defects onconventional optical masks and reticles. In all of these systems,conventional lighting is used and the images are captured by lineararray sensors. These two system choices limit the signal-to-noise ratioand hence the speed of inspection.

SUMMARY OF THE INVENTION

An important object of present invention is to provide a novel defectdetection apparatus which can use both transmitted and reflected lightto inspect a substrate.

Another object of the present invention is to provide a device of thetype described in which surface elevations above a reference elevationare optically determined using interferrometric principals and used asindicators of defects.

Another object of the present invention is to provide a device of thetype described which uses the same optical system to detect defects andmeasure line widths.

Briefly, a preferred embodiment of the present invention includes an XYstage (12) for transporting a substrate (14) under test in a serpentinepath in an XY plane, an optical system (16) including a laser (30), atransmission light detector (34), a reflected light detector (36),optical elements defining reference beam paths and illuminating beampaths between the laser, the substrate and the detectors and anacousto-optical beam scanner (40, 42) for reciprocatingly scanning theilluminating and reference beams relative to the substrate surface, andan electronic control, analysis and display system for controlling theoperation of the stage and optical system and for interpreting andstoring the signals output by the detectors. The apparatus can operatein a die-to-die comparison mode or a die-to-database mode.

One advantage of the present invention is that it uses a laser lightsource and hence has a much higher brightness to scan the mask. Itdiffers from the AMIS system described by Bruning et al. in that itemploys an electro-optical deflection method instead of a mechanicalsystem. Obviously the electro-optical method is faster and more flexiblethan a mechanical device. However, even conventional electro-opticaldeflections do not have sufficient speed to meet system requirements. Inthe present invention the speed is further enhanced by the use of adeflection apparatus previously described for laser beam recording byU.S. Pat. No. 3,851,951 to Jason H. Eveleth, entitled "High ResolutionLaser Beam Recorder with Self-Focusing Acousto-Optic Scanner", issuedDec. 3, 1974.

Another advantage is the use of a stage that has only two degrees offreedom. Prior art also incorporated a rotational capability at aconsiderable cost and complexity. In the present invention the effectivedirection of scanning is controlled by driving both axes of the stagesimultaneously.

Another significant departure from previous art is the ability of thepresent system to simultaneously detect defects with both transmittedand reflected light. This capability is significant because theadditional information can be helpful in determining the nature of thedefect and thereby permits the automatic classification of defects.

Yet another advantage of the present invention is its ability to inspectphase shift masks. It is anticipated that phase shift mask technologywill be used in the 1990's to achieve linewidths of 0.10 micrometers. Inthe present invention the phase shift material can be measured at allpoints on a mask area at the normal scanning speed of the system.

Also advantageous is the ability of the present system to performlinewidth measurement on the mask. This is a significant advantagebecause heretofore two different types of instruments were employed todo both defect detection and linewidth measurement. The ability to use asingle instrument results in a saving of time and, possibly moreimportant, in less handling of the mask, which in turn is significant incontamination control.

A novel feature of the present invention is the autofocusing methodemployed. Previous mask inspection systems used autofocus systems thatwere affected by the pattern on the mask. The present inventionfunctions independently of the pattern.

A significant innovation of the present system is also the two-axispreloading of the stage air bearings. Exceptional stiffness is achievedby this angular loading method.

Also new is the method of correcting for variations of light intensity.In the prior art the spatial non-uniformity of the illumination wasdetermined before an inspection but no provisions existed forcompensating for changing non-uniformity during inspection or, morelikely, variations of the absolute level of intensity during theinspection. In the present invention the intensity is constantlymonitored and immediately compensated in real time. Hence, variations ofthe primary light source with time do not affect the accuracy of theinspection process.

Yet another new capability of the present invention is to inspect themask at substantially the same wave length as used for wafer printing(exposure) through the mask. With advances in technology, increasinglyshorter wavelengths are used for printing. Because the appearance ofdefects changes depending on the wavelength of the illumination, it isimportant to employ approximately the same wavelength light source forboth inspection and printing.

These and other objects and advantages of the present invention will nodoubt become apparent to those skilled in the art after having read thefollowing detailed disclosure of the preferred embodiments illustratedin the several figures of the drawing.

IN THE DRAWING

FIG. 1 is a simplified functional block diagram of a laser maskinspection system in accordance with the present invention;

FIG. 2 is a more detailed schematic representation of the opticalsubsystem depicted in FIG. 1;

FIG. 3 is a diagram illustrating the scanning path used in thedie-to-die inspection mode;

FIG. 4 is a diagram illustrating the scanning path used indie-to-database inspection mode;

FIGS. 5 and 6 are diagrams illustrating possible beam cross sectionsused in the autofocus system;

FIG. 7 is a partially broken perspective drawing illustrating the XYstage;

FIG. 8 is a cross-section taken along the line 8--8 of FIG. 7 showingdetails of the construction frame of the stage;

FIG. 9 is a cross-section taken along the line 9--9 of FIG. 7 showingother details of the construction frame of the stage;

FIG. 10 is an illustration of a cross-section of a typical phase-shiftmask showing in exaggerated scale an illustration of the phase-shiftedoutput of the reflected beam detector;

FIG. 11 is an illustration of the sinusoidally varying detected signalintensity as the mask is scanned in phase shift measurement mode;

FIG. 12 is a block diagram depicting a phase-locked loop subsystem usedto detect phase-shift material thickness;

FIGS. 13a and 13b are simplified schematic diagrams respectivelydepicting operation of the optical subsystem used for measuring thephase-shift material thickness in the transmitted and reflected lightmodes;

FIG. 14 is an illustration used to describe the method of linewidthmeasurement.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawing, a block diagram of an automatic opticalinspection system in accordance with the present invention is shown at10. The system is capable of inspecting substrates, such as reticles,photomasks, semiconductor wafers and phase shift masks, some of whichmay include a plurality of supposedly identical objects.

The system can perform several types of inspection: transmitted lightinspection, reflected light inspection, simultaneous reflected andtransmitted inspection, and phase shift measurement. In transmittedlight inspection, light impinges on the substrate, a photomask forexample, and the amount of light transmitted through the mask isdetected. In reflected light inspection, the light reflecting from asurface of the substrate under test is measured. During phase shiftinspection, the amount of phase shift between two reflected coherentlight beams is detected at each point on the mask while transmittedlight inspection takes place concurrently. The phase shift isproportional to the relative elevation of the surface from which thebeams are reflected. As will be explained below, the transmitted lightsignal is used to qualify the phase-shift signal. In addition to thesedefect detection operations, the system is also capable of performingline width measurement.

In all of the defect detection operations a comparison is made betweentwo images. In die-to-die inspection mode two areas of the substratehaving identical features (dice) are compared with respect to each otherand any substantial discrepancy is flagged as a defect. In thedie-to-database inspection mode a defect is detected by comparing thedie under test with corresponding graphics information obtained from theCADS (computer aided database system) database from which the die wasderived. In the latter case the CADS database is converted to an imageformat as explained in U.S. Pat. No. 4,926,489. (Danielson at al.,"Reticle Inspection System", issued May 15, 1990).

As depicted in the simplified block diagram of FIG. 1, a preferredembodiment of the system 10 is comprised of a stage 12 for carrying asubstrate 14 to be inspected, an optical subsystem 16, a data baseadaptor 18, an electronics subsystem 20, a display 22, a controlcomputer 24 and a keyboard 26.

The Stage

Although a preferred embodiment of the stage 12 will be described indetail below, it suffices at this point to say that the stage is aprecision device driver under control of subsystem 20 and capable ofmoving the substrate 12 under test in a serpentine fashion, within asingle plane, relative to the optical axes of the optical subsystem 16so that all or any selected part of the substrate surface may beinspected.

Optical Subsystem

A detailed block diagram of the optical subsystem 16 is shown in FIG. 2and is essentially a laser scanner apparatus including a light source 30and associated optics which cause a beam 32 of coherent light to bedeflected back and forth over a small angle. As will be furtherdescribed below, the beam sweep is in a direction such that, afterpassing through the optical system, the illustrated spot is swept fromone side to another across the optical axis in a direction generallyparallel to the Y-axis as viewed at the substrate 14 depicted in moredetail in FIG. 3 of the drawing. As the beam is swept, the stage 12carrying the substrate 14 under test is caused to move back and forth inthe direction of the X-axis, being incremented in the Y-direction at theend of each traverse so that the beam 32 is caused to sweep back andforth across a serpentine path 31 passing a plurality of identifiedsubstrate subareas 33, 35, 37 (individual dice in the case of aphotomask) as indicated in FIGS. 3 and 4. In this manner the entiresurface area of the substrate (mask) 14 is swept in a series ofcontiguous swaths 39 by the laser beam. In the case of a transparent orpartially transparent substrate, detection of the image is accomplishedby a transmission detector 34. In the case of a reflective or partiallyreflective substrate, the light reflected from the substrate is detectedby a reflected light detector 36. As will be explained in more detaillater, phase shift mask inspection is carried out by using both of thesedetectors simultaneously.

The light source 30 of the system is a laser, such as the Model5490A5L-00C-115 made by Ion Laser Technology of Salt Lake City, Utah.The light beam 30, emitted by the laser 32, first passes through aspatial filter 38 and is then deflected by the combination of twoacousto optic elements; an acousto-optic prescanner 40 and anacousto-optic scanner 42. These two elements deflect the light beam inthe Y-direction and focus it in the X-direction in a manner similar tothat described in U.S. Pat. No. 3,851,951. (Jason H. Eveleth, "HighResolution Laser Beam Recorder with Self-focusing Acousto-opticScanner", issued Dec. 3, 1974). The deflection system also includes abeam expander 44 and a quarter wave plate

When the beam emerges from the scanner 42, it is convergent in theY-direction, but collimated in the X-direction. A cylindrical lens 50then also focuses the beam in the X-direction, with the focal plane forboth X and Y axes lying at a field stop 52. The beam next passes througha quarter wave plate 54 and a relay lens 56.

The beam is then reflected by a mirror 58, the sole function of which isto fold the optical path. The redirected beam then enters a cube beamsplitter 60 which divides it into paths (different optical axes) 62 and64. The latter path is used only in the phase measurement mode and isotherwise blocked by a shutter 66.

The beam continuing along path 62 is reflected by an oscillating mirror65 which is held fixed during the inspection operation and is used onlyfor displaying an image to an operator on an image display 22 (FIG. 1)during alignment and review. A dove prism 66 is used to rotate thedirection of the scan about the optical axis. The output of prism 66 isfed to one of the telescopes 68 and 70 mounted on a rotatable turret 72.The purpose of these telescopes is to vary the size of the scanning spoton the substrate 14 and thereby allow selection of the minimumdetectable defect size. Since changing the magnification also varies thelength of the scan, the swath width or beam scan width is also changedand therefore the inspection speed is likewise changed. (Only twotelescopes are shown but obviously any number of telescopes, andtherefore spot sizes, can be used.)

From the telescope the beam passes to a mirror 74 and then to a beamsplitter 76 where the path is again split. The reflected portion of beam78 is directed to a detector 80 which serves as a monitor of the beamintensity variation. The unreflected portion of the beam passes throughan objective lens 82 which focuses the beam onto the substrate 14. Lightpassing through the substrate 14 is then collected by a condenser lens84 and a collector lens 86, and focused onto the transmission detector34.

Autofocus Subsystem

The autofocus function is based upon a monitoring of the shape of thelight beam cross-section after it is passed through some anamorphicelements. The basic principal underlying the implementation is that acylindrical lens produces astigmatism. In such a case a focussed beamfirst passes through best focus in one direction and then through bestfocus in the perpendicular direction. In between these two focal pointsalong the beam path the beam cross section is oblong in one directionand transitions along the path through points where the beam crosssection is circular and then oblong in a direction perpendicular to theprevious direction. In this invention the optimum focus of the lightimpinging on the substrate is detected by monitoring the beam crosssection of light reflected from the substrate 14. The shape of the beamcross section is monitored by two silicon quadrature photodiodes 90 and92, such as made by Silicon Detector Corporation of Newbury Park, Calif.

As is explained in more detail below, the actual autofocus systemconsists of two optical paths which differ from each other in thedirection of the astigmation. In one path the cylindrical lens has nocurvature when viewed in the X-direction while in the other path, thecylindrical lens has no curvature in the Y-direction.

The autofocus beam 93 is split off from the reflected beam 95 directedalong reflected detector path by a beam splitter 94, and is redirectedtoward another beam splitter 96 which splits the beam into two paths 98and 100. In FIG. 2 the X-coordinate is perpendicular to the paper andconsequently, cylindrical lens 102 is shown with a curvature, while anidentical element 104, in the other path, appears as a plano-parallelelement. The path leading to detector 90 also contains a spherical lens,106. The two identical quadrature detectors 90 and 92 detect across-section of each beam. As the substrate surface position, orthickness, varies, the beam cross section, as seen by the detectors,varies in the X-direction as shown in FIGS. 5 and 6 at 108, 110 and 108,112 respectively. It is to be noted that on neither detector does thevertical (Y-direction) diameter of the illuminated area change. When themask is in focus, both detectors are illuminated by a circular beam 108.As the mask goes out of focus, the horizontal diameter shrinks on onedetector (see FIG. 5), while on the other one it increases (see FIG. 6)as indicated by the outlines of the beam 110 and 112, respectively. Thischanges the electrical output from the quadrature detectors. The focuscorrection signal F_(c) is then: ##EQU1## where A₁ is the signal derivedfrom quadrants along the X axis of 90,

A₂ is the signal derived from quadrants along the X axis of 92,

B₂ is the signal derived from quadrants along the Y axis of 90,

B₂ is the signal derived from quadrants along the Y axis of 92.

Transmitted Light Inspection Mode

Ordinarily, transmission mode detection is used for defect detection onsubstrates such as conventional optical masks having transparent areasand opaque areas. As the laser beam scans the mask, the light penetratesthe mask at transparent points and is detected by transmitted lightdetector 34 which is located behind the mask 14 and measures the lightcollected by condenser lens 84 and collector lens 86.

Reflected Light Inspection Mode

Reflected light inspection is normally performed on opaque substratesthat contain image information in the form of developed photoresistfeatures. Light reflected by the substrate passes backwards along thesame optical path as described before but is then diverted by apolarizing beam splitter 60 into detector 36. A condenser lens 35projects the light onto the detector 36. As previously stated, duringreflected light inspection, shutter 66 is closed.

Reflected light inspection may also be used to detect contamination ontop of opaque substrate surfaces.

Phase Shift Material Thickness Measurement Mode

The measurement of phase shift is of interest only at points where thesubstrate is transparent, i.e., where there is no opaque geometry. Thepresence of opaque geometry is detected by the transmission detector 34and only in the spaces separating such geometry is a phase shiftmeasurement taken. During this operation shutter 66 is open and lightfrom the laser reflected by splitter 60 travels through relay lenses 110and 112, which form a telescope 114, and through a low numericalaperture objective lens 116 to a tilted mirror 118 where it is reflectedback along the same path and through beam splitters 60 and 94, andcondenser lens 35 to reflected light detector 36. At the same time,detector 36 is also illuminated by light which first passes throughsplitter 60 to be reflected from a point on the substrate and which onreturning is reflected by splitter 60 to the detector. These twoluminous beams interfere with each other, and the intensity of the lightdetected by detector 36 therefore varies as a function of the relativeoptical path length of the two paths 62 and 64. As will be explained inmore detail later, this data is interpreted by the electronic subsystemto determine variations of thickness of phase shift material covering agiven point on the substrate.

Simultaneous Detection by More than one Type of Detector

It is important to note that transmitted and reflected lightinspections, and the phase shift measurement operation are not mutuallyexclusive in time. Simultaneous transmitted and reflected detection candisclose the existence of an opaque defect sensed by the transmitteddetector while the output of the reflected detector can be used todisclose the type of defect. As an example, either a chrome dot or aparticle is opaque and hence will result in a dark output from thetransmission detector, but reflective chrome defects also produce a highreflected light indication while a particle will typically reflect less.By using both reflected and transmitted detection one may locate aparticle on top of chrome geometry. In general, one may determinesignatures for certain types of defects, such as the ratio of theirreflected and transmitted light intensities. This information can thenbe used to automatically classify defects.

Similarly, transmitted light detection and phase shift measurement canoccur simultaneously. On a phase shift mask an opaque defect in a regioncovered by phase-shift material can be detected, and the absence ofopaque material detected by the transmitted light detector 34 can beused to gate the phase shift measurement.

Control Computer

The control computer, 24, acts as the operator console and mastercontroller of the system and is a device such as a SPARC computer madeby Sun Microsystems of Mountain View, Calif. All system interfaces withthe operator and the user's facilities are made through the controlcomputer. Commands are issued to and status is monitored from all othersubsystems so as to facilitate completion of the operator assignedtasks.

Electronics Subsystem

The function of the electronics subsystem 20 is to interpret and executethe commands issued by control computer 24. These functions are:digitize the input from detectors 34 and 36; compensate these readingsfor variations in the incident light intensity; detect defects in theimage and transfer the defect data to the control computer 24;accumulate the output of the interferometers used to track the stage 12;provide the drive for the stages linear motors; and monitor sensorswhich indicate status.

Except for the measurement of phase shift and line width information,all of the enumerated functions of control computer 24 and subsystem 20have been described in the above-mentioned U.S. Pat. Nos. 4,247,203,4,579,455, 4,633,504, 4,805,123, 4,926,489, and 4,644,172. It is to benoted that in the above patents the same functions are performed in manydifferent ways and the particular approach adopted depended on theavailability and suitability of integrated circuit devices at the timethe system was being developed. Any of the cited approaches could beused.

The Stage

The stage is an air-bearing X-Y stage that is driven by a linear motoron each axis. The position of the stage along each axis is monitored byinterferometers (not shown), such as the Model TIPS V, made by TeletracCorporation.

Stage 18 is shown in detail in FIG. 7 with the front rail cut away topermit view of the principal elements. The stage has two degrees offreedom; it has no rotational capability. It is disclosed here forapplication in the described inspection system but could also be used inmicrolithography and any precision machining application.

The Y carriage 120, in the shape of a frame 120, carries the X stage124. The motion of both stages is controlled by linear motors and airbearings. The attractive force between the stator and the slider of eachlinear motor provides the preload of the linear bearings.

The Y carriage frame includes two guideways 126 and 127, controlling themotion of the X stage 124 inside the carriage. The guideways areconnected by two side rails 128. (The front rail, the equivalent of 128,is not shown.) The stator 129 of the X linear motor is imbedded insidethe X guideway 126 in such a way that it attracts the X slider 130attached to air-bearing housings 131 and preloads four of the five X airbearings 132, 133, 134 and 135. A separate magnet 130 and ferromagneticpreload strip 137 provide the preload to air bearing 138. Each bearingis equipped with a swivel, enabling rotation of the bearing pad abouttwo axes, in addition to rotating the bearing itself, thus the onlydegree of freedom constrained by an air bearing is the translation inthe direction normal to the pad surface.

The X stage carries the specimen 14 and is kinematically constrained bythe five air bearings: the bearings 132 and 135 control the pitch of theX stage motion, and constrain the vertical translation in the Zdirection, bearings 133 and 134 control the yaw of the X motion andconstrain the horizontal translation in the Y direction. Bearing 138nested in the housing 139 controls the roll of the X stage andconstrains vertical translation of the stage in the Z direction. Thespecimen holder assembly 140 is attached to a lightweight compositeframe 141 of the X stage.

The stage contains a number of novel features. One such feature is theuse of the linear motor to preload the stage in two directions andthereby achieve an exceptional stiffness. This is accomplished by thearrangement of triangular cross section slider iron 130 and angularposition of the stator 131, so that the magnetic attraction force is atan angle to all four air bearings 132, 133, 134 and 135.

Another innovative feature of the design is that the stator 129 oflinear motor is imbedded inside the guideway 126 at an angle to the twowalls of the guideway.

Also novel is the use of honeycomb material, such as Blue Seal, made byHexcell of Dublin, Calif., for the construction of frame 140. Thisreduces the mass of the stage, yet makes it very rigid. A cross-sectionof this construction taken along the line 8--8 is shown in FIG. 8 wherecellular insert 142 is sandwiched between skins 143. The bottom plate144 and top plate 145 join the skins 143 and complete the box structureenclosing the insert 142. The honeycomb material may be replaced by anynumber of light composite materials, such as Duocell, manufactured byERG of Oakland, Calif.

Also novel is the method of attaching the solid pieces 146 to thecomposite in the way that they penetrate one skin of the composite walland are attached to the opposite skin and either of the top or bottomplates, as shown in FIG. 9, with joints 147 formed around thepenetration through the wall, and between the solid piece and the insideof the opposite skin and the plate 144.

OPERATION OF THE DISCLOSED EMBODIMENT

Alignment

Prior to starting the automatic inspection operation, the operatoraligns the mask in the proper orientation and defines to the computerthe "care area", i.e., the area to be inspected. FIG. 3 illustrates thedesired orientation of the inspection path 31 with respect to dice 33,35, and 37 shown here on a multi-die mask or reticle 14. Duringinspection, the stage 12 is moved in a serpentine manner, following thepath 31, while the laser beam is deflected parallel to the Y-axis of themask. As stage 12 moves in the X-direction, this Y-axis motion of thelaser beam sweeps out a swath, 39. Ordinarily the axes of mask 14 willnot be parallel to the drive axis of the stage. Therefore, an X or a Ydirectional motion of the stage requires both of the drives of the stageto be driven simultaneously. The first task of the operator is thereforeto define to the system the ratio of the speeds of the major axes of thestage. To accomplish this, the operator chooses two points known to himto lie on the same X-coordinate of the die. He then drives the stage tothese points, while observing the image on image display 22. The systemnotes the location of these points by measuring the travel withinterferometers (not shown) along the drive axes of the stage. Thesemeasurements establish the direction cosines of the stage drive axeswith respect to the X and Y axes of the mask. At this time the doveprism 66 (FIG. 2) is rotated to orient the deflection of the laser beamso that it is perpendicular to the X-directional travel of the stage.Next, the operator designates to the system the care area 41 (FIG. 3) ofthe die, the area to be inspected.

Phase Shift Measurement Calibration

For reasons that will become apparent later, in the phase measurementmode, as the laser spot scans (in the Y-direction) a flat transparentsurface parallel to the plane of the mask, the intensity variessinusoidally, as shown by curve 200 in FIG. 11. Mathematically, theintensity I is:

    I=Asin [(2 πy/w)-D)]+I.sub.o                            (2)

where y is the distance of the pixel in question from the origin, w is aconstant that is a function of the tilt angle of mirror 118 (FIG. 2), Dis the phase shift due to path length change as the result of thethickness of the phase shift material, A is the half-amplitude of theintensity, and I_(o) is the intensity offset 204 due to stray light inthe optics. These values are all determined during the phase shiftmeasurement calibration part of the initialization. As the laser scans aflat uniform transparent area of the mask, the intensities at eachpicture element are digitized and stored in the computer. Then, I_(o) isthe average value of the intensities over integer cycles, and A can becomputed from:

    A=(I.sub.max -I.sub.o)/2                                   (3)

The value W is the periodicity of the sinusoid.

It is to be noted that I_(o) and A are different for clear and phaseshift material covered areas and therefore must be determined for bothareas. The quantity D is a linear function of the thickness of the phaseshift material and this relationship is determined by calibration on aknown sample containing various thickness phase shift material featuresand remains constant while the system retains dimensional stability.

The Inspection Process

Automatic inspection of a reticle ordinarily starts at the upper lefthand corner of the care area and follows the serpentine pattern 31. (SeeFIG. 3) As the stage slowly moves in the X direction, the laser beamrapidly sweeps back and forth in the Y-direction. In this manner a swath39 is scanned and the digitized output of the detectors is stored in theelectronics subsystem 20. When the swath reaches the left boundary ofthe care area of the second die 35, image data derived from die 33, andnow stored in subsystem 20, is compared with the data derived from die35. Any substantial difference is designated a defect. In a similarmanner, the data from die 37 is also compared with the data derived fromdie 35.

When the scanning process reaches the right boundary of the care area ofdie 37, the stage is moved in the Y-direction an amount slightly lessthan the swath width and the stage starts a return trace in theX-direction. In this manner the care areas of the dice are traversed bythe serpentine motion.

Die-to-database inspection, ordinarily performed on single die reticles,is similar to die-to-die inspection except that the comparison occursbetween the die and a simulated image generated by database adaptor 18.FIG. 4 illustrates the die-to-database scan path 31'.

Review Operation

After completion of the automatic inspection operations, the operatorreviews the defects by causing control computer 24 to move the stage 12to the area of a particular defect and hold it there. The image is thenscanned by using the acousto-optic scanners 40 and 42 to sweep the laserbeam in the Y-direction, and by using the oscillating mirror 65 to ineffect translate the sweeping beam back and forth in the X-direction,and the digitized image is displayed on display 22. The operator may usethe output of any of the detectors or the combination of outputs frommore than one detector. If the operator desires, the different detectoroutputs may be superimposed and represented as separate colors on thedisplay.

Phase Shift Material Thickness Measurement

FIG. 10 is an illustration of the cross section of one type of a phaseshift mask. While the present example relates to a particular type ofmask, on all types of masks, control of the thickness of phase shiftmaterial is a requirement and hence the technique described here isapplicable to all types of phase shift masks.

The substrate 160 is typically of quartz on which opaque features 164are deposited. These are typically thin layers of chrome. Phase shiftfeatures 161 and 162 made of transparent material will typicallypartially overlay part of the chrome 164 and some of the clear areas 181and 183 between the features 164. Phase shift material filledtransparent areas 181, 183 and clear areas 180, 184 typically alternate.The height of the upper surface 173 of the phase shift feature 162 abovethe level of the front, or upper, surface 174 of the quartz substrate istypically such that it produces a phase shift of 180 degrees withrespect to a point 180 in the same plane but not covered by phase shiftmaterial.

Defects in phase shift masks may occur in several ways. There can bedefects in the transparent areas, such as either excess chrome or dirt,or there can be missing chrome in a feature 164. Such defects aredetected by the transmitted light detector 34 (FIG. 2) and are thesubject of previously referenced prior art. The present invention isalso capable of detecting defects in the phase shift layer 161 or 162.There are two types of defects: those where there is a sudden variationof thickness of the phase shift layer, and those in which there is adeviation from the desired thickness which is either constant, or variesslowly over the surface. The former type of defect, such as the divot168 in layer 161, is detected by the transmitted light detector 34because it scatters the light and hence does not allow the light to passthrough the phase shift material. It therefore appears as a dark spot intransmission. Slowly varying surfaces 172 or incorrect thickness of thephase shift layer, such as depicted in feature 161, are detected byinterferometric methods, as explained below.

A perfectly flat surface, such as 173 at the top of 162, parallel to theplane of the mask and with an optical path length L will produce fringesas the mask is scanned because, due to the tilted mirror 118, thewavefront of the reference beam is not parallel to the plane of thesubstrate. (In order to avoid any ambiguity in the direction of thechange of the phase, the tilt of mirror 118 should be greater than themaximum expected slope of any surface such as 161.) The detector outputin such a case is a sine wave, such as that shown in FIG. 11. A similarflat surface located at a path length L+D (see FIG. 10) will produce asine wave of the same frequency but with a phase shift D with respect tocurve 200. This second sine wave is shown as wave form 202.

As the mask is scanned in the Y-direction, the transmitted lightdetector 34 detects whether a particular pixel is fullly transparent.Only at such fully transparent pixels are reflected light intensitymeasurements taken and digitized. At such pixels, the reflected lightintensity is determined and digitized. This is suggested by thedepiction at the bottom of FIG. 10 wherein it is indicated that duringthe time that the scan is passing across the non-transparent feature164, as determined by the output of detector 34, the output of detector36 is ignored. From the intensity value, and from the Y-coordinate ofthe pixel, together with the values of A, w and I_(o) determined duringthe calibration, electronic subsystem 20 determines d in Equation 2 andthe corresponding path length variation at the pixel, i.e., the heightof the feature surface above plane 174.

It is to be noted that due to the periodic nature of a sinewave, thereis an ambiguity because path length variations corresponding to a phaseshift of 360 degrees are indistinguishable. However, sudden variationsresulting in a 360° phase shift can occur only when the phase shiftmaterial contains a ridge. Such a ridge produces diffraction which isthen detected in the transmission mode. Hence, the ambiguity due to a360° phase shift is resolvable and it is possible to continuously, atevery pixel, track the thickness of the phase shift material.

In practice, the mask substrates are not likely to be perfectly parallelto the image plane, nor is the substrate likely to be perfectly flat.However, these variations are gradual, and on a 5× phase shift mask oneneed consider variations only within a radius of 4-5 microns.Specifically, only the relative phase shift between two adjacentfeatures is important, such as the relative phase shift betweenlocations 180, 162 and 184. These points are likely to be less than 4microns apart.

To determine whether there is a phase error of sufficient magnitude toindicate a defect on the substrate, the path length is computed at eachtransparent pixel covered by phase shift material 162 (FIG. 10). Thisvalue is then compared with the average of the path lengths of twoadjacent points where there is no phase shift material, such as points180 and 184. If the difference in path length differs from an acceptablevalue by more than a predetermined threshold value at the print wavelength, such as 10 degrees for example, the phase shift materialthickness at the inspected point is marked as defective.

In addition to making path length comparisons between points ongeometric features in the same vicinity, the system also checks for amissing or extra geometric feature, such as may occur in the patterngeneration. In die-to-die mode, the path lengths of pixels at 173, 180and 184 (FIG. 10) of the 33 (FIG. 3) are compared with the path lengthsat the corresponding pixels of die 35. This comparison will disclose anymissing geometric features, unless both dice 33 and 35 have the sameerror. Similarly, in die-to-database mode a comparison can be madebetween the path lengths associated with the previously designatedpixels and the description of these pixels in the CADs database.

Alternate Phase Shift Measurement Method

The above measurement technique uses a digital approach to determine therelative optical path length at grid points to determine the phase shiftangle at every point. As explained below, one may also employ an analogmethod to find the phase shift angle.

FIG. 12 illustrates the additional circuitry required by this method forinsertion into the apparatus of FIG. 1 at 208 to determine the phaseshift angle. The analog signal derived from detector 36 is fed to oneinput 209 of an analog phase detector 210 which also obtains anothersignal at 211 from a numerically controlled oscillator 212. A signalproportional to the phase difference between these two signals isconverted to a digital form by an eight bit A/D converter 214 and passedto an encoder 216 and also to a digital low pass filter 218. The digitalfilter 218 and the encoder 216 are gated by a gating signal derived fromdetector 34. The digital filter 218, which functions as an integrator,accepts an input only when detector 34 indicates that the mask istransparent at the inspected point. Encoder 21 accepts the 8-bit outputsignal of the A/D converter 214 and shifts it right one bit. If thepixel is transparent at that point, the encoder inserts a 0 into themost significant position of the digital signal and transmits theremaining signal to subsystem 20 as the phase signal. Should detector 34indicate that the pixel is opaque, the digital signal will be encoded asall ones, 11111111. This signifies to the subsystem 20 that the phasesignal is invalid and should be disregarded.

The previously explained circuitry is a phase-locked-loop that followsslow variations of the phase, as might be caused by slowly varyingphenomena, such as imperfect flatness of the mask. The output of theencoder 216, when valid, indicates the path length variation in thelocal area.

Alternate Phase Shift Optical System Implementation

In some instances it is desirable to measure the actual phase shift,rather than infer the phase shift from the relative path length. Thismay be done by using transmitted interferometry. FIGS. 13a and 13b aresimplified schematic diagrams, in which for simplicity many of theelements shown in FIG. 2 are omitted, but illustrate a variation of thepreferred embodiment that permits measurement in either or both atransmit mode or a reflected mode using respectively transmitted lightinterferometry and simultaneous measurement of the reflected andtransmitted interference pattern.

As depicted in FIG. 13a, to implement this alternative operating in thetransmit mode, a pelicle beam splitter 230 is added which reflects lightreceived from splitter 60 and produces a reference beam at detector 34via the path 231 past tilted mirror 232, objective lens 234 and anotherbeam splitter 236. The interference of the reference beam and theimaging beam passing along path 240 and through substrate 14 is detectedat detector 34.

In the reflected light mode, reference light split by splitter 60 isdirected along the path 250 to tilted mirror 118 and returned todetector 36 where it interferes with imaging light passing throughsplitter 60 and along the path 260 to substrate 20 where it is reflectedback along path 260 and reflected by splitter 60 into detector 36.

It is to be noted that this alternative also permits the simultaneousmeasurement of the phase in both the reflected and transmitted modes.

Because lasers have a limited coherence length in both the reflected andtransmitted interference modes, the path length should be approximatelythe same for the imaging beam path and the reference beam path.

Line Width Measurement

FIG. 14 shows a plan view of a small portion 270 of a mask. Area 272 istransparent and is crossed by a feature 274 that may either be opaque(chrome or other material) or transparent if the quartz substrate of themask is covered by phase shift material. The system measures theintensity at equidistant grid points, depicted at 276. As explained morefully below, these intensity measurements are then used to determine theline width, i.e., the distance 278 across feature 274.

It is to be noted that at each of the grid points 276 the intensity isthe convolution of the point spread function of the optical system withthe transmissivity profile of the feature. Typically, the transmissivityprofile is a step function. Therefore, for a straight feature, as isshown in FIG. 14, the intensity measured at a particular grid point is afunction of the perpendicular distance from the grid point to the edgeof the feature (line 274). The intensity at a particular point in thevicinity of a feature can thus be interpreted as the perpendiculardistance from the point to the line. This interpretation is done in asimple table look-up operation in the computer 24 (FIG. 1). On the basisof the intensities at grid points 280 and 282, distances S₁ and S₂ areknown and the slope of the edge relative to a feature is: ##EQU2## wherea is the distance between the grid points 280 and 282 and G is angle284.

Once the slope of the edge of a feature (line) has been determined, theopposite edge of the line can be similarly located, and a verificationcan be made that it is parallel to the previously calculated line edge.On the basis of the intensities along the two edges of the line, thelinewidth is calculated in control computer 24.

The previously described method of line measurement is, strictlyspeaking, normally applicable only to conventional masks which have nosurface areas covered by phase shift material. However, the techniquedescribed above may also be used for the measurement of phase shiftfeatures because, at the boundary between a clear area and an areacovered by phase shift material, defraction of the incident light beamwill occur and along this narrow boundary no light will be transmitted.The line width is the distance between the center of one boundary andthe center of the opposite boundary.

Although the present invention has been described above in terms ofpreferred embodiments, it is anticipated that various alterations andmodifications thereof will be apparent to those skilled in the art. Forexample, to avoid the need to sweep the laser beam during the scanningoperation, instead of using the linear detector 34 in the preferredembodiment, one could use a time delay integrating sensor of the typedescribed in the above-referenced Levy U.S. Pat. No. 4,579,455. Withsuch modification, if a laser is used as the light source, coherence inthe Y-direction would have to be destroyed by using a rotating groundglass. The coherence in the X-direction is destroyed by the time delayintegrating sensor. It is therefore intended that the following claimsbe interpreted as covering all such alterations and modifications asfall within the true spirit and scope of the invention.

What is claimed is:
 1. An optical inspection system for inspectingobjects formed on substrates selected from the group consisting ofphotomasks, reticles, phase shift masks and semiconductor wafers,comprising:stage means for carrying a substrate to be inspected suchthat a surface of said substrate moves in a particular manner within aninspection plane; laser means for providing a pixel illuminating beam oflight; optical means defining a first optical axis intersecting saidinspection plane and along which said pixel illuminating beam of lightis initially passed, said optical means including a variablemagnification subsystem for focusing said beam of light to a pixeldefining spot on the substrate to be inspected; beam deflecting meansdisposed along said first optical axis and operative to deflect saidbeam of light in oscillatory fashion whereby said pixel defining spot iscaused to sweep across the surface of said substrate from one side toanother of the path traced by the intersection of said optical axis withsaid substrate as said substrate is moved in said particular manner, andin a direction transverse to said path, as the substrate is carriedalong said path, the limits of deflection of said beam of light from oneside of said path to the other defining the width of a scanning swathover a care area of the substrate including at least a portion of one ofsaid objects; light detecting means for detecting changes in theintensity of said beam of light caused by its intersection with theinspected substrate as said beam of light is either transmitted orreflected by said substrate, said light detecting means being responsiveto the detected changes in intensity and operative to develop scansignals corresponding thereto, said light detecting means includingsampling means for sampling said scan signals to produce pixel samplesignals; and electronic means for comparing said pixel sample signals tocorresponding reference signals whereby differences therebetween may beused to identify defects in the inspected substrate.
 2. An opticalinspection system for inspecting objects formed on substrates such asphotomasks, reticles, phase shift masks and semiconductor wafers,comprising:stage means for carrying a substrate to be inspected suchthat a surface of said substrate moves in a particular manner within aninspection plane; laser means for providing a pixel illuminating beam oflight; optical means defining a first optical axis intersecting saidinspection plane and along which said pixel illuminating beam of lightis initially passed, said optical means being operative to focus saidbeam of light to illuminate a pixel defining spot on the substrate to beinspected, the spot size determining at least one dimension of a pixelof the substrate; beam deflecting means disposed along said firstoptical axis and operative to deflect said beam of light in oscillatoryfashion whereby said pixel defining spot is caused to sweep across thesurface of said substrate from one side to another of a path traced bythe intersection of said optical axis with said substrate as saidsubstrate is moved in said particular manner, and in a directiontransverse to said path as the substrate is carried along said path, thelimits of deflection of said beam of light from one side of said path tothe other defining the width of a scanning swath over care areas of thesubstrate including at least a portion of one of said objects; lightdetecting means for detecting changes in the intensity of said beam oflight caused by its intersection with pixel areas of the inspectedsubstrate as said beam of light is either transmitted or reflected bysaid substrate, said light detecting means being responsive to thedetected changes in intensity and operative to develop scan signalscorresponding thereto; electronic means for comparing said scan signalsto corresponding reference signals whereby differences therebetween maybe used to identify defects in the inspected substrate; means forrecording the substrate locations of said defects; means forsubsequently causing said stage means to reposition and hold theinspected substrate at a previously recorded location of a selecteddefect; oscillatory reflective means disposed along said optical axisfor causing a portion of said optical axis to sweep back and forth alonga portion of said path in cooperation with said beam deflecting meanswhereby a selected segment of a swath is repetitively scanned and saidlight detecting means continuously generates display signalsrepresentative of the selected defect; and display means responsive tosaid display signals and operative to provide a visual display ofsubstrate area covered by the swath segment and including the selecteddefect.
 3. An optical inspection system for inspecting objects formed onsubstrates selected from the group consisting of photomasks, reticles,phase shift masks and semiconductor wafers, comprising:laser means forproviding a pixel illuminating beam of light; optical means defining afirst optical axis along which said pixel illuminating beam of light isto be passed; stage means for moving a substrate to be inspected suchthat a surface thereof is carried within an inspection plane intersectedby said optical axis and such that the point of intersection traces aserpentine path over at least a portion of the substrate including atleast one of the objects to be inspected; beam deflecting means disposedalong said first optical axis and operative to deflect said pixelilluminating beam of light in oscillatory fashion such that said beam oflight sweeps back and forth across said path from one side to anotherand in a direction generally transverse to said path such that, as thesubstrate is carried, the deflection of said beam of light from one sideof said path to the other defines a scanning swath over the portion ofthe substrate; light detecting means for detecting changes in theintensity of said beam of light caused by pixel areas of the inspectedsubstrate illuminated by said beam of light as said beam of light iseither transmitted or reflected by said substrate, said light, detectingmeans being responsive to the detected changes in intensity andoperative to develop scan signals corresponding thereto; and electronicmeans for comparing said scan signals to corresponding reference signalswhereby differences therebetween may be used to identify defects in theinspected substrate, and for recording the location of said defects onsaid substrate.
 4. An optical inspection system for inspecting objectsformed on substrates selected from the group consisting of photomasks,reticles, phase shift masks and semiconductor wafers including aplurality of supposedly identical patterned objects disposed in anordered array on a surface thereof, comprising:laser means for providinga pixel illuminating beam of light; optical means defining a firstoptical axis along which said pixel illuminating beam of light is to beinitially passed; stage means for moving a substrate to be inspectedsuch that a surface thereof moves within an inspection plane intersectedby said optical axis and such that as the substrate is moved the pointof intersection traces a serpentine path over a portion of the substratesurface including at least one of the objects; beam deflecting meansdisposed along said first optical axis and operative to deflect saidbeam of light in oscillatory fashion such that said beam of light sweepsback and forth across said path from one side to another and in adirection generally transverse to said path such that, as the substrateis carried, the deflection of said beam of light from one side of saidpath to the other defines a scanning swath across the portion of thesubstrate; light detecting means for detecting changes in the intensityof said beam of light caused by pixel areas of the inspected substrateilluminated by said beam of light as said beam of light is eithertransmitted or reflected by said substrate, said light detecting meansbeing responsive to the detected changes in intensity and operative todevelop scan signals corresponding thereto; electronic means for storingfirst scan signals developed as said scanning swath passes over a firstof the patterned objects and for comparing the stored first scan signalsto second scan signals developed as the scanning swath passes over asecond of the patterned objects whereby differences therebetween may beused to identify defects in the inspected substrate; and means forrecording the locations of said defects on said substrate.
 5. An opticalinspection system as recited in any one of claims 1 through 4 whereinsaid stage means is constrained to move with no more than two degrees offreedom, and is driven by x-direction and y-direction inputs generatedby said electronic means.
 6. An optical inspection system as recited inclaim 5 wherein said stage means is an X/Y air bearing stage driven bylinear motors including stators and sliders separated by air bearingmeans, the stators being configured to preload the air bearings of eachslider in at least two directions.
 7. An optical inspection system asrecited in any one of claims 1 through 4 wherein said laser means isselected to have a predetermined wavelength of substantially the samewavelength as a light source to be later used in association with theinspected substrates during wafer processing.
 8. An optical inspectionsystem as recited in any one of claims 1 through 4 wherein said opticalmeans includes means for rotating the direction of deflection of saidbeam of light so that it has a desired relationship to saidpredetermined path.
 9. An optical inspection system as recited in anyone of claims 1 through 4 wherein said optical means also includes meansdefining a second optical axis along which said beam of light may bepassed, and having a portion thereof in common with a portion of saidfirst optical axis, the optical path lengths of said first and secondoptical axes being substantially equal so that the two axes experiencesubstantially identical thermal variation, said second optical axisincluding a tilted mirror for reflecting the beam of light back alongsaid second optical axis and onto said light detecting means forinterferometric comparison with light reflected back along said firstoptical axis from the substrate under inspection.
 10. An opticalinspection system as recited in any one of claims 1 through 4 whereinsaid light-detecting means includes a first detector for detecting theintensity of light reflected from said substrate and for generatingfirst scan signals commensurate therewith.
 11. An optical inspectionsystem as recited in claim 10 wherein said light detecting means furtherincludes a second detector for detecting the intensity of light fromsaid beam passing through the substrate under inspection and forgenerating second scan signals commensurate therewith.
 12. An opticalinspection system as recited in claim 11 wherein said electronic meanscompares said first and second scan signals to first and secondreference signals and determines both the existence and type of anydefect encountered.
 13. An optical inspection system as recited in anyone of claims 1 through 4 wherein said light detecting means includes atransmitted light detector for detecting the intensity of light fromsaid beam of light passing through the substrate under inspection andfor generating corresponding scan signals.
 14. An optical inspectionsystem as recited in any one of claims 1 through 4 wherein said lightdetecting means includes means for monitoring the intensity of said beamof light and for generating an output which may be used by saidelectronic means to provide beam intensity variation correction to saidscan signals.
 15. An optical inspection system as recited in any one ofclaims 1 through 4 wherein said beam deflecting means includes anacousto-optic modulator for causing said beam of light to be deflectedback and forth over a relatively small angle, said angle being thefactor which determines the width of said scanning swath.
 16. An opticalinspection system as recited in any one of claims 1 through 3 andfurther comprising database means containing ideal data corresponding tothe substrate under inspection, said data being selectively read out togenerate said reference signals.
 17. An optical inspection system asrecited in any one of claims 1 through 3 and further comprising meansfor storing said scan signals, the stored signals being subsequentlyused to provide said reference signals to which presently scannedsignals may be compared.
 18. An optical inspection system as recited inany one of claims 1 through 4 wherein said light detecting meansincludes a first detector for detecting light transmitted through thesubstrate being inspected and a second detector for simultaneouslydetecting light reflected from the substrate being inspected, said firstand second detectors respectively generating first and second signalsfrom which said electronic means can determine both the existence of adefect and the type of defect detected.
 19. An optical inspection systemas recited in any one of claims 1 through 4 and further comprising meansdisposed along a second optical axis intersecting said first opticalaxis and operative to develop a beam of light for interfering with lightreflected from the substrate under inspection to develop an interferencebeam for detection by said light detecting means such that the intensityof the interference beam detected by said light detecting means may beused by said electronic means to determine variations in the height ofthe inspected surface material above a particular surface of theinspected substrate.
 20. An optical inspection system as recited in anyone of claims 1 through 4 and further comprising means defining anauto-focus optical axis intersecting said first optical axis andincluding means for introducing astigmatism in the beam reflected fromthe surface of the inspected substrate, and auto-focus detecting meansfor measuring the shape of the astigmatized beam and determiningtherefrom the degree of focus of the beam on the substrate, saidauto-focus detecting means generating correction signals for input tosaid electronic means.
 21. An optical inspection system as recited inclaim 20 and further comprising means for splitting said auto-focusoptical axis into first and second branches having oppositely polarizedanamorphous elements which distort the shape of the light beam passedtherethrough, and wherein said auto-focus detecting means includes afirst image shape detector associated with said first branch and asecond image shape detector associated with said second branch, theoutputs of said image shape detectors being used by said electronicmeans to determine whether said beam of light is in focus at saidinspection plane, and if not in focus to determine the degree anddirection in which the beam is out of focus.
 22. An optical inspectionsystem as recited in claim 21 wherein said first and second image shapedetectors are appropriately oriented quadrature detectors.
 23. Anoptical inspection system as recited in any one of claims 1 through 4wherein said electronic means measures the magnitudes of said scansignals corresponding to predetermined grid points on the surface of thesubstrate under inspection and then uses such measurements to determinethe surface dimensions of substrate features.
 24. An optical inspectionsystem as recited in claim 17 wherein said light-detecting meansincludes a first detector for detecting the intensity of light reflectedfrom the substrate under inspection and for generating first scansignals commensurate therewith.
 25. An optical inspection system asrecited in claim 24 wherein said light detecting means further includesa second detector for detecting the intensity of light from said beampassing through the substrate under inspection and for generating secondscan signals commensurate therewith.
 26. An optical inspection system asrecited in claim 25 wherein said electronic means compares said firstand second scan signals to corresponding first and second referencesignals and determines both the existence and type of any defectencountered.
 27. An optical inspection system as recited in claim 26wherein said light detecting means includes means for monitoring theintensity of said beam of light and generating an output which may beused by said electronic means to provide beam intensity variationcorrection to said scan signals.
 28. An optical inspection system asrecited in claim 27 and further comprising means defining an auto-focusoptical axis intersecting said first optical axis and including meansfor introducing astigmatism in the beam reflected from the surface ofthe inspected substrate, and auto-focus detecting means for measuringthe shape of the astigmatized beam and determining therefrom the degreeof focus of the beam on the substrate, said auto-focus detecting meansgenerating correction signals for input to said electronic means.
 29. Anoptical inspection system as recited in claim 28 and further comprisingmeans for splitting said auto-focus optical axis into first and secondbranches having oppositely polarized anamorphous elements which distortthe shape of the light beam passed therethrough, and wherein saidauto-focus detecting means includes a first image shape detectorassociated with said first branch and a second image shape detectorassociated with said second branch, the outputs of said image shapedetectors being used by said electronic means to determine whether saidbeam of light is in focus at said inspection plane, and if not in focusto determine the degree and direction in which the beam is out of focus.30. An optical inspection system as recited in claim 29 wherein saidfirst and second image shape detectors are appropriately orientedquadrature detectors.
 31. An optical inspection system as recited inclaim 30 wherein said electronic means measures the magnitude of saidscan signals corresponding to predetermined grid points on the surfaceof the substrate under inspection and then uses such measurements todetermine the surface dimensions of substrate features.
 32. An opticalinspection system as recited in claim 31 wherein said stage means is anX/Y air bearing stage driven by linear motors including stators andsliders separated by air bearing means, the stators being configured topreload the air bearings of each slider in at least two directions. 33.An optical inspection system as recited in claim 32 wherein said opticalmeans includes means to rotate the direction of deflection of said beamof light so that it has a desired relationship to said predeterminedpath.
 34. An optical inspection system as recited in claim 16 whereinsaid light-detecting means includes a first detector for detecting theintensity of light reflected from said substrate and generating firstscan signals commensurate therewith.
 35. An optical inspection system asrecited in claim 34 wherein said light detecting means further includesa second detector for detecting the intensity of light from said beampassing through the substrate under inspection and for generating secondscan signals commensurate therewith.
 36. An optical inspection system asrecited in claim 35 wherein said electronic means compares said firstand second scan signals to first and second reference signals anddetermines both the existence and type of any defect encountered.
 37. Anoptical inspection system as recited in claim 36 and further comprisingmeans defining an auto-focus optical axis intersecting said firstoptical axis and including means for introducing astigmatism in the beamreflected from the surface of the inspected substrate, and auto-focusdetecting means for measuring the shape of the astigmatized beam anddetermining therefrom the degree of focus of the beam on the substrate,said auto-focus detecting means generating correction signals for inputto said electronic means.
 38. An optical inspection system as recited inclaim 37 and further comprising means for splitting said auto-focusoptical axis into first and second branches having oppositely polarizedanamorphous elements which distort the shape of the light beam passedtherethrough, and wherein said auto-focus detecting means includes afirst image shape detector associated with said first branch and asecond image shape detector associated with said second branch, theoutputs of said image shape detectors being used by said electronicmeans to determine whether said beam of light is in focus at saidinspection plane, and if not in focus to determine the degree anddirection in which the beam is out of focus.
 39. An optical inspectionsystem as recited in claim 38 wherein said electronic means measures themagnitude of said scan signals corresponding to predetermined gridpoints on the surface of the substrate under inspection and then usessuch measurements to determine the surface dimensions of substratefeatures.
 40. An optical inspection system as recited in claim 39wherein said stage means is an X/Y air bearing stage driven by linearmotors including stators and sliders separated by air bearing means, thestators being configured to preload the air bearings of each slider inat least two directions.
 41. An optical inspection system as recited inany one of claims 1 through 4 wherein said light detecting meansincludes a transmission detector disposed along said first optical axison the side of said inspection plane opposite the side including saidlaser means and operative to detect transmitted light passing through aninspected substrate, and wherein said optical means further includesmeans defining a second optical axis along which said beam of light maybe passed, said second optical axis not intersecting said inspectedsubstrate but having at least a portion thereof in common with saidfirst optical axis and intersecting said transmission detector wherebyinterferometric comparison between said beam of light and saidtransmitted light may be conducted.
 42. An optical inspection system asrecited in claim 41 wherein said optical means also includes meansdefining a third optical axis along which said beam of light may bepassed, and having a portion thereof in common with a portion of saidfirst optical axis, the optical path lengths of said second and thirdoptical axes being substantially equal so that the two axes experiencesubstantially identical thermal variation, said third optical axisincluding a tilted mirror for spatially shifting and reflecting thespatially shifted beam of light back along said third optical axis andonto said reflection detector for interferometric comparison with lightreflected along said first optical axis from the inspected substrate.43. An optical inspection system as recited in claim 11 and furtherincluding circuit means responsive to said first and second scan signalsand operative to develop gated signals for input to said electronicmeans to indicate defects in the inspected substrate as a function ofthe phase error between the light reflected from the surface of thesubstrate and the light passing through the inspected substrate.
 44. Anoptical inspection system as recited in claim 43 wherein said circuitmeans includes:an analog phase detector for comparing said first scansignal to a reference signal to develop an analog signal proportional tothe phase difference therebetween, means for converting said analogsignal to a digital signal, and encoder means gated by said second scansignal and operative to generate said gated signals.
 45. A method ofinspecting objects formed on substrates selected from the groupconsisting of photomasks, reticles, phase shift masks and semiconductorwafers, comprising the steps of:transporting a substrate such that asurface thereof to be inspected moves within an inspection plane;providing means defining a first optical axis intersecting saidinspection plane; directing a beam of light along said first opticalaxis and focusing said beam of light to illuminate a spot of a selectedsize on the substrate to be inspected; sweeping said beam of light inoscillatory fashion such that the illuminated spot moves from one sideto another of a path defined by the intersection of the optical axiswith the substrate surface as said substrate surface is moved withinsaid inspection plane whereby care areas of the substrate including atleast one of said objects are scanned in swaths the width of which isdetermined by the beam sweep limits; detecting changes in the intensityof said beam of light caused by its intersection with pixel areas of theinspected substrate as said beam of light is either transmitted orreflected by said substrate, and developing scan signals correspondingthereto; sampling said scan signals to produce pixel sample signals; andcomparing said pixel sample signals to corresponding reference signalsand using differences therebetween to identify defects in the inspectedsubstrate.
 46. A method of inspecting objects formed on substratesselected from the group consisting of photomasks, reticles, phase shiftmasks and semiconductor wafers, comprising the steps of:transporting asubstrate to be inspected such that a surface thereof moves within aninspection plane; providing a pixel illuminating beam of light;providing optical means defining a first optical axis intersecting saidinspection plane, the intersection of said first optical axis and saidsubstrate surface describing an inspection path across said substrate assaid substrate is moved; directing said pixel illuminating beam of lightalong said optical axis; selectively focusing said beam of light toilluminate a spot on the substrate surface to be inspected, the spotsize determining at least one dimension of a pixel of the substrate;causing said beam of light to sweep in oscillatory fashion across thesurface of said substrate from one side to another of said path and in adirection generally transverse to said path as the substrate is carriedalong said path, the limits of deflection of said beam of light from oneside of said path to the other defining the width of a scanning swathover a care area of the substrate including at least a portion of one ofsaid objects; detecting changes in the intensity of said beam of lightcaused by its intersection with pixel areas of the inspected substrateas said beam of light is either transmitted or reflected by saidsubstrate, and developing scan signals corresponding thereto; samplingsaid scan signals to produce pixel sample signals; and comparing saidpixel sample signals to corresponding reference signals wherebydifferences therebetween may be used to identify defects in theinspected substrate.
 47. A method of inspecting objects formed onsubstrates such as photomasks, reticles, phase shift masks andsemiconductor wafers, comprising the steps of:transporting a substrateto be inspected such that a surface thereof moves within an inspectionplane; using a laser means to provide a pixel illuminating beam oflight; providing an optical means defining a first optical axisintersecting said inspection plane, the intersection of said firstoptical axis and said substrate surface describing an inspection pathacross said substrate as said substrate is moved; directing said pixelilluminating beam of light along said first optical axis; focusing saidbeam of light to illuminate a pixel defining spot on the substrate to beinspected, the spot size determining at least one dimension of a pixelof the substrate; causing said beam of light to be deflected inoscillatory fashion such that said spot moves across the surface of saidsubstrate from one side of said path to another and in a directiontransverse to said path as the substrate is moved, the limits ofdeflection of said beam of light from one side of said path to the otherdefining the width of a scanning swath over care areas of the substrateincluding at least a portion of one of said objects; detecting changesin the intensity of said beam of light caused by its intersection withpixel areas of the inspected substrate as said beam of light is eithertransmitted or reflected by said substrate, and developing scan signalscorresponding thereto; comparing said scan signals to correspondingreference signals and using differences therebetween to identify defectsin the inspected substrate; recording the locations of said defects onsaid substrate; subsequently causing said stage means to reposition theinspected substrate such that a previously recorded location of aparticular defect is intersected by said optical axis; stopping saidstage means to hold the repositioned substrate in place; sweeping aportion of said optical axis back and forth along said path in adirection orthogonal to the deflection caused by said beam deflectingmeans so as to cause said light detecting means to continuously generatedisplay signals representative of the substrate surface area in theimmediate vicinity of said selected defect; and using said displaysignals to provide a visual display of the substrate surface includingthe selected defect.
 48. A method of inspecting objects formed onsubstrates selected from the group consisting of photomasks, reticles,phase shift masks and semiconductor wafers, comprising the stepsof:using a laser means to provide a pixel illuminating beam of light;providing optical means defining a first optical axis along which saidpixel illuminating beam of light is passed to illuminate a single pixelarea of a substrate to be inspected; moving the substrate to beinspected within an inspection plane intersected by said first opticalaxis such that the point of intersection traces a serpentine path overat least a portion of the substrate including at least one of theobjects to be inspected; deflecting said beam of light in oscillatoryfashion such that the illuminated pixel area moves from one side of saidpath to another and in a direction generally transverse thereto and suchthat, as the substrate is carried, the deflection of said beam of lightfrom one side of said path to the other defines a scanning swath overthe portion of the substrate; detecting changes in the intensity of saidbeam of light caused by pixel areas of the inspected substrateilluminated by said beam of light, as said beam of light is eithertransmitted or reflected by said substrate, and developing scan signalscorresponding thereto; and comparing said scan signals to correspondingreference signals whereby differences therebetween may be used toidentify defects in the inspected substrate, and recording the locationof said defects on said substrate.
 49. A method of inspecting objectsformed on substrates selected from the group consisting of photomasks,reticles, phase shift masks and semiconductor wafers including aplurality of supposedly identical patterned objects disposed in anordered array on a surface thereof, comprising the steps of:using alaser means to provide a pixel illuminating beam of light; using anoptical means to define a first optical axis along which said pixelilluminating beam of light is passed; using a stage means to move asubstrate to be inspected within an inspection plane intersected by saidoptical axis such that the point of intersection traces a serpentinepath over at least a portion of the substrate including at least one ofthe objects to be inspected; deflecting said beam of light inoscillatory fashion to illuminate pixels on a first one side of saidpath and then another, and in a direction transverse to said path suchthat, as the substrate is carried along said path, the deflection ofsaid beam of light from one side of said path to the other defines ascanning swath over the portion of the substrate; detecting changes inthe intensity of said beam of light caused by pixel areas of theinspected substrate illuminated by said beam of light as said beam oflight is either transmitted or reflected by said substrate, anddeveloping scan signals corresponding thereto; storing first scansignals developed as said scanning swath passes over a first of thepatterned objects and comparing the stored first scan signals to secondscan signals developed as the scanning swath passes over a second of thepatterned objects, and using differences therebetween to identifydefects in the inspected substrate; and recording the locations of saiddefects on said substrate.